Prime Minister Jacinda Ardern says that the NZ Transport Agency dropped the ball over Auckland's light rail plans and caused the current delay.

And it appears that the two city-to-airport proposals currently being considered are now light rail versus heavy rail, though Ardern would not be drawn on any details, saying they are commercially sensitive.

She also declined to comment on whether the cost of Auckland rail to the airport might be estimated to cost up to $10 billion, more than twice the original estimate of $4b and far in excess of the current estimate of $6b.

The Government is currently considering two bids for the project: one from NZ Infra, a joint venture between the New Zealand Super Fund and Canada's CDPQ Infra group, and one from the NZTA.

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Labour campaigned in 2017 on building light rail in Auckland from the city to the airport, estimating at the time that it would cost $4b.

But concerns have since been raised about cost blowouts; Deputy Prime Minister Winston Peters said yesterday that he had been told by a credible source in the transport sector to be on the alert for cost blowouts.

Stuff reported from leaked documents that the NZ Infra proposal looked more like heavy rail, with two underground stations on Queen St and elevated sections over Mt Eden.

The Ministry of Transport has called the leaked documents outdated, and Ardern would not be drawn on details of the proposals this morning.

Newstalk ZB's Mike Hosking said of the NZ Infra proposal: "Its driverless trains over hill and down dale to the tune of $10 billion."

Ardern said: "We haven't finished the process of assessing the proposal, so I'm not going to bandy around numbers until that is completed."

But she said that NZTA had dropped the ball and caused delays.

"We did lose some time because Cabinet asked NZTA to go through a process with the NZ Infra proposal."

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Hosking: "But they didn't, because they're useless."

Ardern: "I was going to jump in there, but actually I won't. So we have transferred that over to the Ministry of Transport and Treasury and they're now undertaking that process."

Ardern told Radio NZ that NZTA had messed up.

"Cabinet asked them to do a proper assessment of both proposals. They did not. That caused a delay. We've now said that NZTA shouldn't be the one assessing."

She said Labour's original signal of a $4b cost came from the NZTA proposal.

"We now have a substantively different proposal in front of us. I'm not saying it would be more or less. At this point, we are still sizing and developing and assessing the proposal."

She said there was no cost blow out because there was no confirmed project yet.

Asked about whether the NZ Infra proposal was more about heavy rail than light rail, Ardern did not deny it.

"I've always described the NZ Infra proposal as being substantively different both in terms of the financing of it, but also some of the detail of the operation side."

But she would not be drawn on any details.

"While we're assessing a proposal, it's not for me to give away the detail of someone in the middle of being assessed as a potential developer of this project."

The Government remained committed to the project, she said.

"We absolutely believe we have to build rail from the city centre out into the south to Māngere. That is how we're going to have a world-class city that deals with congestion."

NZTA has been contacted for comment.